One of the serious challenges the semiconductor industry faces is developing new gate dielectric materials for DRAM and capacitors. For decades, silicon dioxide (SiO2) was a reliable dielectric, but as transistors have continued to shrink and the technology has moved from “Full Si” transistors to “Metal Gate/High-k” transistors, the reliability of the SiO2-based gate dielectric is reaching its physical limits. The need for new high dielectric constant materials and processes is increasing and becoming more and more critical as the size for current technology shrinks.
Standard dielectric materials like TiO2 or new dielectric materials containing alkaline earth metals called strontium titanates, such as SrTiO3 or Sr2TiO4, or barium strontium titanates provide a significant advantage in capacitance compared to conventional dielectric materials. The new dielectric materials are also attractive candidates for several thin film applications, such as high dielectric constant materials for electronic devices, anti-reflection optical coatings, biocompatible coatings, photocatalysis, and solar cells. (H. A. Durand et al., Appl. Surf. Sci. 86, 122 (1995); C.-W. Wang of al., J. Appl. Phys. 91, 9198 (2002); M. Keshmiri et al., J. Non-Cryst. Solids 324, 289 (2003); T. Inoue et al., Nature (London) 277, 637 (1979); H. Kim et al., Appl. Phys. Lett. 85, 64 (2004)).
In addition, TiO2 is also a constituent of several important multi-metal oxide systems, such as strontium titanates (STOs), barium strontium titanates (BSTs), and lead zirconium titanates (PZTs), for dielectric and ferroelectric applications. (P. Alluri et al., Integr. Ferroelectr., 21, 305 (1998); J. F. Scott et al., Science 246, 1400 (1989)).
Nevertheless, deposition of Ti containing layers is difficult and new materials and processes are needed. For instance, atomic layer deposition, ALD, has been identified as an important thin film growth technique for microelectronics manufacturing, relying on sequential and saturating surface reactions of alternatively applied precursors, separated by inert gas purging. Frequently, an oxygen source such as ozone or water is used in this deposition method. The surface-controlled nature of ALD enables the growth of thin films of high conformality and uniformity with an accurate thickness control.
In STO ALD deposition, available Sr precursors show excellent reactivity with O3 and acceptable reactivity with water. However, the use of ozone as an oxidant may have undesired results with the underlying layer, such as TiN or strontium ruthenium oxide (SRO), when the STO film is deposited at high temperature. It may either oxidize the TiN layer or partially etch Ru from SRO layer.
Although atomic layer deposition (ALD) of Ti compounds has been disclosed, these metal precursors have poor reactivity, especially with moisture, and low stability often requiring low substrate temperatures and strong oxidizers to grow a film, which is often contaminated with carbon or nitrogen.
Air Liquide showed that most of the standard homoleptic Ti molecules have limited ALD process temperature window or no deposition (R. Katamreddy, V. Omarjee, B. Feist, C. Dussarrat, ECS conference 2008). For example, in a water ALD process, the Ti molecules titanium tetrakis(isopropoxide) (TTIP), tetrakis(dimethylamino) titanium (TDMAT), tetrakis(diethylamino) titanium (TDEAT), and tetrakis(ethylmethylamino) titanium (TEMAT) had deposition rates below 0.6 Å/cycle and process windows that did not exceed 250° C. Id.
New Ti precursors having higher thermal stability at higher process temperatures are needed. High temperature processes are desired to generate high quality TiO2 (doped or undoped) and STO films with very high dielectric constants (preferably with k≧50). It has been reported that an STO film with a dense and columnar polycrystalline microstructure and a small average grain size (30 nm) is required to obtain a low leakage current with a high k value (C. S. Hwang, S. O. Park, C. S. Kang, H. Cho, H. Kang, S. T. Ahn, and M. Y. Lee, Jpn. J. Appl. Phys., Part 1, 34, 5178 1995).
Zhang et al. disclose the unexpected synthesis of Ti(Cy-NC(NiPr2) N-Cy)2(OnBu)2. Chinese Science Bulletin (2005), 50(24), 2817-2820. Chen et al. disclose the synthesis of Ti(OnBu)2(O2CMe)2. Huaxue Xuebao (2003), 61(10), 1592-1596. Uses for these compounds were not disclosed.
US Pat App Pub No 2005/277223 discloses ALD methods of forming metal oxides using metal-containing precursors having the formula M(L1)x(L2)y, wherein M is a metal, L1 and L2 may be halide, diketonate, alkoxide, amino, alkoxyamine, amidinate, or multidentate ligands. The exemplary precursors however are only Hf(OtBu)2(NEtMe)2, Hf(OtBu)2(NEt2)2, Hf(NEt2)2(DMAMP)2, Hf(NEtMe)2(DMAMP)2, Ti(OtBu)3Cl, Ti(OtBu)3Me, Ti(OtBu)2(NEt2)2, Ti(NEt2)2(DMAMP)2, Ti(OtBu)2(DMAMP)2, and TiCl2(DMAMP)2.
Therefore, a need remains for precursors suitable for titanium/H2O ALD processes compatible with Sr ALD process.
New chemical vapor deposition (CVD) processes are also required for Ti materials. Other sources and methods of incorporating Ti materials are being sought for new generations of integrated circuit devices. Novel precursors are needed.